feat: add a post-install script to symlink the common files
refactor: improve post-install script
fix: now support to specify a branch name while using this package

For example, it is now possible to add it like so:

`npm install --save git+https://github.com/karma-runner/common.git#post-install-script`
fix: remove the package.json recursive search in post-install script

The post-install script will now only look three directories above to create the
symlinks. It will no longer recursively search for a corresponding
`package.json`.
Merge pull request #4 from karma-runner/post-install-script

Post install script